Steel tire cords, such as the kind used in steel-belted tires, may be manufactured from a plurality of core filaments which are wrapped in a plurality of sheath filaments. More core filaments are required to achieve higher strengths, but when three or more core filaments are required, the core filaments tend to bunch together and form a void in the center of the bunched filaments. When the cord is bonded in a layer of rubber, the rubber cannot easily penetrate into and fill the voids. If the tire is then perforated, water may enter the voids and corrode the tire cord.
Recent tire designs require thinner rubber gauges and/or wider cord spacing in the belts in order to produce lighter weight tires. These designs are known to be better for automobile fuel efficiency and ride quality.
In addition, passenger car tires require cords that provide lateral maneuverability, i.e., good cornering, and low bending stiffness for ride comfort and maximum contact with the road surface. To achieve the desired lateral stiffness, larger diameter filaments are typically used for construction of the tire cord. As these diameters increase to improve cornering, the tire belts become stiffer in the vertical plane causing uncomfortable ride and smaller tire contact area with the road.
Typical tire construction uses tire cords having 0.15–0.40 mm brass plated steel filaments. If high breaking loads are required of a cord, an increase in the number of filaments is necessary. When cords are stranded with three or more filaments, a void may be created in the center of the cord. The cord then does not have enough space between filaments to allow rubber to penetrate into the void during tire curing and the cord may suffer from reduced adhesion. A reduction in adhesion may result in the tire having a belt separation. Furthermore, if a cord has a void center, it may be corroded easily by water if the belt area is penetrated by any road hazard. This is especially a concern since the full length of cord in the belt may be corroded by water wicking through the void space. The resulting corrosion degrades the mechanical properties and the fatigue resistance of the cord such that tire failure may occur.
Various tire cord constructions have been developed to improve the rubber penetration into the cord and to avoid the problem of voids within the tire cord.
(1) Open constructions are those created by pre-forming a large amplitude wave into the filaments with a frequency equal to the cord pitch to create a small elongation spring-type cord.
(2) Wavy filament constructions have one or more small wave filaments, whose pitch is smaller than the cord's pitch. The small openings created by this construction allow rubber to penetrate the core.
(3) The tire cord constructions known as 1×2 and 2+2 are completely rubber penetrated.
Information relevant to other attempts to address the problems described above can be found in various U.S. Patents as described following.
U.S. Pat. No. 5,718,783 to Ikehara discloses a steel cord comprising a single helical core filament and five to eight sheath filaments. The pitch and the amplitude of the helical core filament are set within certain ranges depending upon the diameter of the core filament and the number of sheath filaments.
U.S. Pat. No. 6,244,318 to Shoyama discloses a tire cord formed from a single core filament and a plurality of sheath layers formed by helical windings about the core.
U.S. Pat. No. 6,089,293 to Niderost discloses a rubber ply in which the reinforcing cords have different properties at different points in the ply. The differing properties are achieved by having the cords twisted together helically and having different helical diameters in different parts of the ply.
U.S. Pat. No. 3,802,982 to Aldefer discloses a tire where the reinforcing is provided by a plurality of helically formed single filament cords.
U.S. Pat. No. 5,285,623 to Baillievier et al. disclose a steel cord comprising two strands of at least two filaments each. The strands are twisted about each other forming helicoids of the same pitch. The filaments of one of the strands has a pitch of more than 300 mm, i.e., the filaments of this stand are not twisted to any significant degree.
However, all of these tire cords have some limitations. The filaments of open constructions can move easily because they are loosely stranded. Therefore, it is difficult to keep a stable cord shape and the cord basically is not uniform. Tension control in the calendaring process is also very important for open cords and must be kept low in order to allow good rubber penetration. If open constructions are used in high-tension calendars, the openness along with rubber penetration may be lost when the cord closes during elongation. Problems with calendar sheet rubber gauge are also evident with open cords. The larger openness that is required to assure rubber penetration also increases the cord's diameter and requires an increased rubber gauge to accommodate the size of the cord.
Wavy constructions are less effective in keeping good rubber penetration in higher strength applications where an increase in the number of filaments is required in a cord. This is because the wavy construction creates relatively small openings in the cord and, as a result, do not allow for large amounts of rubber flow during curing.
The cord types known as 1×2 and 2+2 also present problems for tire design because they require a fixed number of filaments. Larger filament diameters, higher tensile steels, or even increased EPI (ends per inch) in the calendar must be utilized to get higher strength for tire belts. Larger filament diameters and higher EPI both are contrary to lightweight tire design.
More recently, another construction has been introduced to meet the requirements of rubber penetration into the cords and lateral belt stability in tires. This construction, as disclosed in Japanese Published Patent Application 2000-096464, uses all parallel filaments that are wrapped with a single, thin, low strength wrapping filament. However, actual production of this construction is difficult because the low-strength wrapping filament cannot keep the parallel core filaments from flaring unless a very short wrapping pitch is used. This reduces production output because wrapping machine speeds are constrained by a maximum machine RPM. Furthermore, this construction has difficulty keeping a large number of filaments flat because the wrapping filament does not have enough strength to hold the filaments flat. Higher breakload cords are unavailable since the number of filaments is limited. In addition, the thin wrapping filament does not contribute significantly to the breaking strength of the cord.
The limitations of the prior art are overcome by the present invention as described below.